1. Field of the Invention
This invention relates to improvements in and relating to a tapered leaf spring element for vehicle axle shaft suspensions, which has a reduced thickness towards opposite ends thereof, and to a laminated leaf spring comprising a plural number of such leaf spring elements.
2. Description of the Prior Art
Conventional leaf springs of this sort have a shape which is flat in a cross-section taken along a plane perpendicular to the lengths thereof so that, upon the exertion of a bending moment, concentrated stress occurs in corner edge portions of the loaded surface on the tension side, which can cause fatigue failure in these portions.
In the production of leaf springs for vehicle axle shaft suspensions, it has been the general practice to use ordinary flat rolled steel strips which have a substantially rectangular shape in a cross-section taken along a plane perpendicular to the longitudinal direction of the steel strip. In some cases there is employed a flat rolled steel strip of a trapezoidal shape, or a channel-like shape having a groove in the bottom surface in a cross-section taken along a plane perpendicular to the length of the steel strip. The reason that steel plates of the above-mentioned sectional shapes are used for the leaf spring resides in that the steel strip of such a sectional shape has a broader width on the tension side than on the compression side, in contrast with the steel strip of plain rectangular cross-section, so that the neutral axis of the sectional area is shifted toward the tension side under a bending load, thereby reducing the tension (increasing the compression) under a bending moment and increasing the amplitude of the bearable repeated bending moment per unit weight. For example, a laminated leaf spring comprising a number of leaf elements of the grooved or trapezoidal cross-sectional shape, which is widely used in vehicular suspensions, is subjected to a mean bending moment due to a static load as well as a variable amplitude bending moment due to a dynamic load. Improvements in the fatigue strength per unit weight of such springs have been attempted utilizing the fact that the fatigue strength is higher on the compression side of the beam than on the tension side under a mean bending moment due to a static load.
However, in contrast with the steel strip of simple rectangular cross-sectional shape, greater difficulties are involved in rolling a leaf element of a cross-sectional shape as shown in FIG. 2 or 3 within a prescribed tolerance, in addition to increases in the rolling cost. Further, leaf elements having conventional cross-sectional shapes are disadvantageous in that fatigue fracture is apt to occur at the corner edge portions of the transverse cross-sectional shape on the tension side where a concentration of stress takes place, exhibiting a fatigue strength about 20% lower than that of a round bar spring (a spring having round cross-sectional shape) which has no such critical edge portion.
In order to resolve these problems it has been proposed to provide a leaf spring element of a particular cross-sectional shape which is designed to lower the stress on the tension side as compared with that on the compression side, to preclude stress concentration at the corner edge portions.
U.S. patent application Ser. No. 256,784 filed on Apr. 23, 1981 continuation application Ser. No. 518,766 filed on Aug. 1, 1983 in which three of applicants are the same as in this application discloses a leaf spring element which has a circularly arcuate shape with a convex surface on the tension side and a concave surface on the compression side in a transverse cross-section, and in which the neutral axis is shifted toward the tension side to reduce the tensile stress under a bending moment.
However, as is clear from beam theory, the bending stress acting on the leaf spring varies along the length thereof, and the stress is smaller at a portion, farther from a center portion of unit length thereof where the stress is maximum. Since the arcuate cross-sectional shape of the proposed leaf spring is the same throughout the length thereof and the size of the cross-section is determined by the maximum bending stress, the thickness of the leaf spring becomes excessive at some portions thereof, causing the weight of the leaf spring to be excessively large.
On the other hand, a leaf spring whose width is constant and whose cross-sectional area varies along the length thereof with the thickness around the center portion being a maximum has been proposed, as shown in FIG. 1. With this leaf spring 1, the distribution of bending stress is made uniform throughout the length thereof. This leaf spring is also effective in weight reduction. That is, compared with a conventional leaf spring having the same cross-sectional area throughout its length, the leaf spring 1 allows a weight reduction of about 10 to 15% while maintaining the strength thereof and attaining uniform stress distribution.
However, since the cross-sectional shape of this spring is rectangular throughout its length, the fatique breakdown is still concentrated at edge portions 1b in the upper surface 1a. That is, compared with an edgeless rod spring of the same material, the fatigue strength of the leaf spring 1 is less by about 20%. Therefore, the leaf spring 1 does not satisfy all of the requirements as to bending stiffness per unit weight, fatigue strength per unit weight and total weight reduction.